I posted this message last year and never got any responses on it.
Since the subject has come up again, I will repeat it and see if anyone
has any good information. Am I missing anything here? The message follows:
The way most people do it is to put a stainless steel shim between the
zinc tower and the copper wire and clamp it. Polyphaser makes clamps
that work like that.
That said, doing it this way doesn't make any sense to me. You have
three goals in avoiding galvanic corrosion.
1) Maximize the contact area of the least noble metal exposed to the
electrolyte (water),
2) Minimize the potential difference between the metals in contact, and
3) Keep the water out.
1) The least noble metal in this case is the zinc tower, but the contact
area is the same for both metals because water may collect within the
junction but the whole tower is not going to emersed in a liquid. So
item 1 (in the case of what is in contact with the tower) is a moot point.
2) The least nobel metal will corrode. If you look at the small list
below, you will see this is the zinc (your tower- not good). In order to
avoid this you should put something next to your tower as closely
matched to the tower as possible. If that corrodes you can always
replace it.
In the galvanic series, in flowing sea water, in order from least noble
to more noble:
zinc is around -.98 to -1.03
aluminum alloys are -.76 to -1.0
copper is -.3 to -.37
tin is -.3 to -.33 and
stainless steel varies a lot depending upon the alloy and may be -.57 to
0.0.
Looking at this table, I can't figure out why people use stainless steel
next the their towers. I would think an aluminum alloy would be a better
choice. After going through this exercise, I decided to use one of the
standard AC entrance panel aluminum alloy ground lugs (Home depot,
Lowes, or any electrical house). I think some of these may be tin
plated, others are not. Don't use the tin plated ones. I used an
oversized clamp because I liked the robust clamping screw it had. I bent
the end of my #4 ground wire into a tight J loop, which I placed under
the screw to gain more surface contact area. When you tighten the
clamping screw, the copper wire actually squashes into the aluminum a
little. These clamps work well under a tower bolt (of a crank-up) after
you file off the little ridge on the bottom of the clamp, and you will
probably have to drill the mounting hole larger.
I coated the aluminum alloy to copper junction with an anti-oxidant and
sealed it against water. I used duct seal to encapsulate the whole thing.
Jerry, K4SAV
---------------------
Gene Smar wrote:
>John:
>
> Most of us here on TT recommend your using solid rather than stranded #2.
> This will eliminate corrosion from between strands, maintaining the
> electrical properties of the grounding wire.
>
> As for connecting to the tower, you might consider connecting to the flat
> steel elements at the base of the tower using these model 213 clamps from
> Harger: http://www.harger.com/products/grdcmp/mech/bc/bc.cfm . I use them
> to connect my shunt-feed wires and my coax shield ground wires to the flat
> elements of my Trylon tower. I also use their model 222T clamps
>http://www.harger.com/products/lpcmp/blp/bl/BL/bl.cfm to connect my ground
> wires to pre-drilled holes in the Trylon's legs.
>
> A final word of advice: You ought to connect your ground wires higher up
> along the tower than right atop the concrete base. This right-angle turn in
> the grounding path represents a greater inductance to lightning energy than a
> gentle curve you get from connecting up higher along the tower legs. You can
> protect this piece of ground wire with a piece of garden hose that extends a
> few inches into the dirt and above the top of the concrete base.
>
>73 de
>Gene Smar AD3F
>
>
>
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